The CMB gives us critical information about our cosmic past. But it doesn’t give us everything, and galaxy mapping can fill in a key gap.
Imagine you want to know about the Universe, and what its properties were like when it was first born. There are two ways to go about it: the “clean” way and the “messy” way. The clean way involves looking back to the earliest possible cosmic times — i.e., the stages where early, primordial signals are most pristine — and look for the relics of the signals as they were back in those early stages. However, not every piece of information is obtainable in this “clean” fashion; sometimes, all we can do is look at the signals that exist at late, modern times in our Universe, which leaves us with the “messy” option: look for the imprints of those early signals in the modern, late-time data that we can acquire today.
When we look for a “clean” signal, it’s harder to do better than the CMB, or cosmic microwave background. It represents a specific event in cosmic history: the transition from the Universe being an ionized plasma — full of electrons, atomic nuclei, and photons — to being full of those same photons, but with neutral atoms instead of ionized, charged particles. This powerful probe of the early…
